Bicyclopropyl systems

Although the gem-dihalocyclopropanes are fairly stable compounds, they can participate — as has been shown in the above sections — in quite a number of chemical transformations. Several reactions between dihalocarbenes and alkenes have been described in which no dihalocyclopropane formation could be observed that these intermediates might have been produced was only inferred from the type of products finally isolated. A typical process of this type is the e/ufo-addition of dihalocarbenes to norbomene and norbomadiene as discussed above. Comparable rearrangements have been observed, when dichlorocarbene additions either lead to aromatic products or when they cycloadd to rather inert aromatic systems. In the latter case a ring-enlargement takes place. A reaction related to the concerted opening of two cyclopropane rings in a bicyclopropyl system as discussed above takes place when dichlorocarbene is added to spiro[2.4]hepta-4,6-diene [227]. 

The bicyclopropyl product is the major product when the two alkene units are in the same molecule, particularly with cyclohexa-1,4-diene systems which are fairly rigid (equation 51) . Once again the reaction is effective whether the alkene (equation 51) or the alkyne (equation 52) absorbs the light initially. 

Numerous other conversions of 5-(2,2 -bipyridyl)-3,3,7,7-tetramethyl-tran -5-nickelatri-cyclo[4.1.0.0 ]heptane (9) have been reported. Upon treatment with maleic anhydride, tetra-cyanoethene or triphenyl phosphite, 3,3,6,6-tetramethyl-trani -tricyclo[3.1.0.0 ]hexane was formed. With triphenyl phosphite, bi(2,2-dimethylcyclopropyl) and l-(2,2-dimethylcyclo-propyl)-3-methylbuta-l,3-diene were obtained as additional products. Similarly, bi(2,2-dimethylcyclopropyl) was obtained upon acidolysis of the metallacycle. The bicyclopropyl was also released with triethylaluminum. Reaction with phenol gave an intermediate complex 3-[2,2 -bipyridyl(phenoxy)nickela-l,T-bi(2,2 -dimethylcyclopropyl). Dibromomethane reaction of the nickelacyclopentane 9 gave the tricyclo[3.1.0.0 ]hexane system and 3,3,7,7-tetramethyl-rrani-tricyclo[4.1.0.0 ]heptane. On the other hand, treatment of the nickelacyclopentane 9 with trifluoroborane gives 3,3,6,6-tetramethylcyclohexa-l,4-diene. ... 

One of the most striking examples of this type is silver-salt promoted rearrangement of cubane, homocubane, secocubane and similar systems where a bicyclo[2.2.0]hexane system is converted to a bicyclopropyl group. The early results are compiled in several reviews which include controversial mechanistic discussions. 

Similar results of metal-mediated cyclopropane ring opening have been obtained with saturated polycyclic hydrocarbons containing cyclopropane subunits. Prominent examples are bicyclopropyl (I) and similar systems such as quadricyclane (2 tetracyclo[3.2.0.0 . 0 ]-heptane), tetracyclo[3.3.I.0. 0 ]nonane (3), bicyclo[1.1.0]butane (4) and other bicyclo[n.l.O]alkanes 5, spiropentane (6) and other spirocyclopropanecycloalkanes 7. ... 

Tri-, tetra- to polycyclic compounds (part B) appear in the structural formula index in order of ring size (carbocyclic before heterocyclic ring compounds) under appropriate compound headings. Part C contains spiro compounds with similar ordering followed by part D which contains assemblies of identical cyclic systems (c.g., bicyclopropyl). 

The (conjugative) interaction of cyclopropane FCM orbitals with the other a orbitals of the same hydrocarbon molecule, in particular with other FCM orbitals if it is polycyclic and contains further three-membered rings, depends strongly on the topography of the system, e.g. on the twist angle between two directly linked cyclopropane moieties, such as those in bicyclopropyl 118. Therefore the individual references pertaining to the... 

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